In the fabrication of electron devices in the surface of a silicon wafer, contact openings are formed through a dielectric layer, such as SiO.sub.2, to specific portions of the wafer surface. The SiO.sub.2 layer is typically about 200 nanometers (nm) thick and may be formed by chemical vapor deposition as disclosed by Kern in U.S. Pat. No. 3,481,781, incorporated herein by reference. The contact openings must have a smoothly varying continuous surface on which to deposit the contact and interconnect metallization, otherwise openings will occur in the metallization. To provide this smoothly varying, continuous surface it has been found useful to deposit a phosphosilicate glass (hereinafter PSG), typically containing about 6 weight percent of phosphorus, over the dielectric layer, form the contact openings through both layers and then heat the wafer to the softening temperature of the PSG. The PSG softens and flows, thereby forming the smoothly varying surface.
However, during the heating process, phosphorus can diffuse out of the PSG into the SiO.sub.2 layer or into the underlying silicon itself. To prevent this from happening, a contact opening is formed through the SiO.sub.2 layer using well-known photolithographic techniques and chemical etching using, for example, hydrofluoric acid or buffered hydrofluoric acid. A Si.sub.3 N.sub.4 protective layer is deposited over the SiO.sub.2 layer and the exposed surface silicon, and PSG is deposited over the Si.sub.3 N.sub.4 layer as disclosed by Dawson et al. in U.S. Pat. No. 4,273,805, incorporated herein by reference. An opening is formed through the PSG layer over the original opening in the SiO.sub.2 layer by chemical etching in a liquid solution and the wafer is heated to about 1100.degree. C. to allow the PSG to flow. The Si.sub.3 N.sub.4 layer is then removed by a selective liquid etchant which does not affect either the PSG, SiO.sub.2 or silicon. However, in the process of removing the Si.sub.3 N.sub.4 layer, portions of the Si.sub.3 N.sub.4 layer underlying the PSG layer are also removed, leaving a recess in the otherwise smooth wall of the contact opening.
A solution to this problem would be to etch back the PSG where it overhangs the Si.sub.3 N.sub.4 layer. However, known etchants for removal of PSG are not highly selective and also remove SiO.sub.2 from under the Si.sub.3 N.sub.4 layer with the result being that the removal of one undercut produces a second one. Thus, it would be desirable to have a selective etchant which preferentially etches PSG at a much greater rate than it etches SiO.sub.2 or Si.sub.3 N.sub.4.